Formation of planes facilitating thermionic emission

ABSTRACT

A method for improving emission from the surfaces of cathodes in electron-emission devices such as thermionic converters in which a large flux of atoms or molecules of material is condensed upon a substrate to produce a surface bounded primarily by crystallographic planes of maximum atom density. The flux may be produced by evaporation from a source which need have no particular crystal orientation and which is held at a higher temperature than the substrate, the substrate being at a temperature which is a fraction of the melting point of the material being evaporated. In the case of body-centered cubic metals such as tungsten the planes produced are (110) planes. In some instances, it is desirable to roughen the surface of the substrate as by abrasion or sand blasting to facilitate the development of the planes of high atom density and accordingly, favorable work function.

United States Patent Van Someren [451 Jan. 18, 1972 [72] Inventor: Laurence Van Someren, Wayland, Mass.

Thermo Electron Corporation, Waltham, Mass.

[22] Filed: Nov. 18, 1968 [21] Appl.No.: 776,763

[73] Assignee:

[52] US. Cl ..lI7/2l3,117/227, 117/107,

2,539,096 l/l951 Miller..... 2,775,809 l/1957 Steinitz....

Millis ..29/195 Price ..29/198 Primary Examiner-William L. Jarvis Attorney-Kenway, Jenney & Hildreth [5 7] ABSTRACT A method for improving emission from the surfaces of cathodes in electron emission devices such as thermionic converters in which a large flux of atoms or molecules of material is condensed upon a substrate to produce a surface bounded primarily by crystallographic planes of maximum atom density. The flux may be produced by evaporation from a source which need have no particular crystal orientation and which is held at a higher temperature than the substrate, the substrate being at a temperature which is a fraction of the melting point of the material being evaporated. In the case of body-centered cubic metals such as tungsten the planes produced are (l 10) planes. In some instances, it is desirable to roughen the surface of the substrate as by abrasion or sand blasting to facilitate the development of the planes of high atom density and accordingly, favorable work function.

4 Claims, No Drawings IFGKMATHGN (BF PLANES FAClLllTATllNG TllilERlVlllUNllC EMHSSION it is common practice in preparing cathode surfaces for electron-emission devices such as thermionic converters to employ an alkali metal vapor on the cathode. One of the more commonly used materials is cesium vapor. The function of the cesium vapor is to provide an adsorbed layer on the cathode to reduce the effective work function for electron emission, thereby increasing electron emission at a given temperature or achieving a higher emission current at lower temperatures.

The current emitted from the surface of a metal crystal also varies according to the work function of the surface in the presence or the absence of adsorbed layers. In the case of bare tungsten, for one example, it is well known that the (1 l) planes of the crystal have low electron emission or high work function. However, there is a reversal of this condition when the surface is covered with a partial monolayer of adsorbed alkali metal. That is, the (1 l0) planes of the crystal have high electron emission and low work function when so covered. Similar relationships between work function and crystal orientation exist for composite adsorbed layers such as oxygen and cesium.

Also, of course, it is desirable that the surface of an emitter be of uniform crystal structure in order that the emission from that surface may be homogenous. in thermionic energy converters, as in other power-producing devices successful operation is, as a practical matter, dependent upon an emitter surface which has a low work function in the presence of cesium. The particular crystal face in body-centered cubic crystals which best satisfies the requirements is the (l 10) face. it is, therefore, a primary object of the present invention to improve emitters for thermionic converters and other power producing devices.

it is a further object of the present invention to improve the operating characteristics of thermionic energy converters.

it is a still further object of the present invention to obtain a surface on an emitter composed of body-centered cubic crystal material which is uniformly covered with l 10) crystal faces.

Generally, the objects of the invention are attained by the condensation of a large flux of atoms or molecules of material upon a substrate. Conditions of temperature and vapor pressure are so arranged that such a large flux of atoms or molecules impinges upon the substrate to form a surface in which atom density is maximized. Generally, such a surface is composed of many small flat planes bounded by straight lines. The planes are (l 10) planes where body-centered cubic metals are being condensed. The flux may be produced by evaporation from a source having no particular crystal orientation but held at a higher temperature than the substrate. Attainment of the desired improved emitter surface is facilitated in some situations by a roughening of the surface of the substrate prior to the evaporation of the emitter material onto the substrate.

in its broadest aspects, the invention is useful in developing crystal surfaces of a selected type for practically all metals. The description of a preferred embodiment of the invention which follows relates to tungsten, but the principles are applicable to all metals and may, for example, be extended to metals such as rhenium where the planes developed would be (001) planes and iron where the planes would be (1 l 1) planes. The point in common with all metals is that the plane which is developed is the plane of maximum atom density. By defining the practice of the invention in terms of vapor pressure rather than temperatures, the disclosure is applicable to substantially all metals.

However, in view of the almost universal selection of tungsten as the emitter material in thermionic converters, the detailed description is limited to that metal. A substrate which may be a flat or a cylindrical member is placed adjacent a second or donor member. The donor member is, of course, also a flat member when the substrate is flat and may be a rod where is is desired to create the desired surface within a cylindrical member. Where the surface is wanted on the outside of a cylindrical member, the donor is in the form of a concentric sleeve surrounding the cylindrical member.

Both donor and substrate are placed in a vacuumtight enclosure along with a heat source. The spacing between donor 5 and substrate is not critical, although for efficiency of opera tion it is generally preferred to maintain a spacing of l to H) mm. Greater spacings are feasible, but the time necessary to evaporate material is unnecessarily lengthened. The heat source may be of any convenient type capable of heating the donor metal to a temperature at which its vapor pressure is in the range of to 10 torr. The vacuum enclosure is pumped down to about 10 torr, or lower pressure.

Depending upon the material, spacing from the donor, type of heating used on the donor and configuration of donor and substrate, it may be necessary to heat or to cool the substrate. Whichever the case, the substrate is held at a temperature between 0.65 and 0.85 times the melting point of the condensing material on the Absolute Temperature Scale (degrees Kelvin) or in any case at a temperature such that the vapor pressure of the substrate is no greater than one tenth of that of the evaporating donor material.

One form of heat source which lends itself to use within the vacuum enclosure is an electron bombardment system. Although, as indicated above for purposes of general definition, the temperatures are preferably defined in terms of vapor pressure, the source, in the case of tungsten, should be heated to a temperature lying in the range of 2,700 and 3,250 C. Obviously, the achievement of such temperatures requires a heat source of a somewhat specialized nature.

For best results, and in line with the general case noted above, substrate temperatures should be between 0.68 and 0.77 times the absolute melting point in degrees Kelvin of the material being evaporated. Thus, in the case of tungsten, the substrate would be at a temperature of the order of 2,250 to 2,500 C. and its vapor pressure would be approximately 10' to 10* torr. The deposition process is continued until a deposit for a thickness of l to 100 micrometers is formed on the substrate. The period of time involved in producing surfaces of such thickness ranges from ten minutesto a few hours, depending on the source temperature used.

It has been noted above that it is sometimes desirable to sandblast the surface of the substrate prior to exposing it to the high flux of atoms or molecules of the donor material. In point of fact, the technique of such sandblasting is not critical and may be carried out with conventional, industrial coarse sandblasting apparatus. The sandblasting step is of particular usefulness at the lower end of the range of donor temperatures 5 or vapor pressures. it has proven to be unnecessary at the upper end of that range, a substrate surface which has only been ground flat has proved satisfactory.

When the evaporation process is concluded, it is found that the donor material has condensed to form a surface composed of many small flat planes bounded by straight lines. In the case of tungsten the planes are l 10) planes as they would be with other body-centered cubic metals similarly condensed. These surfaces have been found not to change shape or deteriorate even after prolonged exposure to temperatures suitable for thermionic emission, generally as high as 2, l 00 K. or more.

In tabular form, the optimum conditions for forming emitter surfaces in accordance with the present invention may be outlined as follows:

LII

Tm=Melting point of condensing material in degrees Kelvin (absolute temperature scale).

Although what has been described constitutes preferred embodiments of the present invention, various alternatives will suggest themselves to those skilled in the art upon a reading of the foregoing. Such alternatives are believed to be within the purview of the present invention, which should be limited only by the spirit and scope of the appended claims.

What is claimed is:

l. The method of preparing a surface to serve as an emitter of electrons which comprises the steps of sandblasting a surface of a tungsten substrate, placing said surface adjacent, parallel to and at a distance of approximately 1-10 mm. from a second piece of tungsten, enclosing said substrate and said second piece of tungsten in a vacuumtight container, evacuating said container to a pressure less than 10 torr, heating said second piece of tungsten to a temperature at which its vapor pressure lies in the range of approximately -10? substrate and evaporated material to cool.

2. In the preparation of an electron-emitting metallic cathode for an electronic device, the method of forming electron-emissive material in crystallographic planes of maximum atom density upon a substrate which comprises the steps of spacing a quantity of electron-emissive material from a substrate within an evacuated enclosure, heating said electronemissive material to a temperature sufficiently high to form a vapor of said material, maintaining said substrate at a temperature sufficiently high that the vapor pressure of said substrate is no greater than one-tenth of that of said material, and condensing a large flux of said material upon said substrate.

3. [n a method as defined in claim 2, the steps which comprise evaporation of said material from a source held ata temperature at which its vapor pressure is approximately l0 -10 torr and depositing said evaporated material upon a substrate held at a temperature which lies between 0.65 and 0.85 Tm where Tm is the melting point in degrees Kelvin of said material.

4. in a method as defined in claim 3, the further step of abrading the surface of said substrate prior to depositing said material thereon. 

2. In the preparation of an electron-emitting metallic cathode for an electronic device, the method of forming electron-emissive material in crystallographic planes of maximum atom density upon a substrate which comprises the steps of spacing a quantity of electron-emissive material from a substrate within an evacuated enclosure, heating said electron-emissive material to a temperature sufficiently high to form a vapor of said material, maintaining said substrate at a temperature sufficiently high that the vapor pressure of said substrate is no greater than one-tenth of that of said material, and condensing a large flux of said material upon said substrate.
 3. In a method as defined in claim 2, the steps which comprise evaporation of said material from a source held at a temperature at which its vapor pressure is approximately 10 4- 10 2 torr and depositing said evaporated material upon a substrate held at a temperature which lies between 0.65 and 0.85 Tm where Tm is the melting point in degrees Kelvin of said material.
 4. In a method as defined in claim 3, the further step of abrading the surface of said substrate prior to depositing said material thereon. 